Currently, I am using Permatex Extend or similar rust converter, brushes on clear and converts rust to a black, metal-protective coating. Leaves a tough coating you can paint over. I use this stuff under the hood and on the underbody wherever I see rust. I have to do this every year because the rust breaks out again. I just finished doing my underbody, including the oil pan above the exaust where it was pretty rusty. I had to clean the oil pan with spray on degreaser before using the rust converter.

On the body door jambs, etc, I use brush on rustoleum of the same color as the vehicle.

Anybody got a better techniques?